Mr. Spencer, 58, played Leo McGarry, a veteran politician who in recent months had become a vice-presidential candidate on the series about a Democratic-run White House. In parallels to Mr. Spencer's own life, Leo struggled with alcoholism and suffered a massive heart attack over the course of the seven-season drama. He won an Emmy Award in 2002 for his work; he was nominated five times.

The plot trajectory of the current "West Wing" season is expected to be reimagined to reference the death of Leo, but any such announcement is premature, according to the producers. There has been speculation that this was to be the final season for the series, but no one expected that the plot would be impacted by the loss of a cherished actor.

Mr. Spencer was no stranger to the stage. A native New Yorker who was raised by blue collar parents in New Jersey, he appeared in regional theatres in Broadway's Execution of Justice and was a warm, folksy presence in a number of Off-Broadway plays, including Warren Leight's Glimmer, Glimmer & Shine, in which he played a burnt-out jazz musician, and The Day Room (for Manhattan Theatre Club), Still Life (American Place Theatre), Peter Hedges' Good As New (MCC Theater), The Ballad of Soapy Smith (The Public Theater), El Salvador and Amulets Against the Dragon Forces (Circle Repertory Theatre) and more.

Mr. Spencer won an Obie Award for the 1981 production of Still Life, about a Vietnam veteran, and received a Drama Desk nomination for The Day Room.

As a child, Mr. Spencer attended the Professional Children's School in Manhattan. His cohorts there included Liza Minnelli and violinist Pinchas Zukerman. He later attended Farleigh Dickinson University. Among his early jobs was a recurring role as the boyfriend of English cousin Cathy on "The Patty Duke Show." One of his major film appearances was playing a detective in the Harrison Ford picture "Presumed Innocent" in 1990. He then became a regular on TV's "L.A. Law."